Patient safety is being put at risk by overtired and stressed out doctors, according to a new report.
The study, from the Royal College of Physicians (RCP), found huge issues with rota gaps in hospitals around the country and some doctors so busy they have no time to get a drink of water.
Many are discouraged from taking naps on night shifts despite evidence they would perform far more safely after a short sleep, it said.
The findings are part of a wider report on how the NHS is suffering due to an overstretched workforce, a recruitment crisis and underfunding.
The RCP is calling for the Government to increase NHS funding or hold an honest debate with patients about what can and cannot be provided by the health service.
Even with planned efficiency savings, the NHS faces a massive funding gap as it tries to tackle an ageing population, cuts to social care and growing demand, the report said.
It found that most doctors (85%) believe that current health funding is insufficient, with NHS funding set to increase by just 0.2% per year in real terms to 2020.
The RCP argues there are no longer enough doctors "to staff our hospitals safely".
The report added: "Between 2013 and 2015, the number of doctor vacancies increased by 60%. Seven out of 10 doctors in training now report working on a rota with a permanent gap. Over half of doctors in training report that rota gaps are having a serious or extremely serious impact on patient care.
"Hospital consultants tell a similar story. According to new, unpublished data from the 2015/16 consultant census, eight in 10 consultants report gaps in the rotas of doctors in training; more than one in four report gaps so serious and frequent that they cause significant problems for patient safety."
The study said NHS staff "increasingly feel like collateral damage" in the battle between rising demand and squeezed budgets, and argued the situation is "untenable".
A quarter of doctors in training said their working pattern leaves them feeling short of sleep on a daily or weekly basis. The report said: "Yet, despite compelling evidence showing the benefits to patient safety, many doctors are actively discouraged from taking naps during night shifts. Three-quarters (74%) of doctors in training go through at least one shift per month with insufficient hydration, and over one-third (37%) do not drink enough water on seven shifts per month.
"Over one-quarter (28%) have worked four shifts per month without a meal, while over half (56%) have worked at least one shift per month without a meal."
On average, doctors work an extra five weeks per year on top of their rostered hours, the report added.
It said that even with planned efficiency savings - which it argues are largely unachievable - the NHS faces around a £2.5 billion black hole in its finances.
It concluded: "The NHS needs a new plan – no more quick fixes or temporary solutions, but a plan designed to meet the UK's health and care needs in the long-term."
RCP president, Professor Jane Dacre, said: "As a doctor, I realise that this is a tough diagnosis for the NHS. However, a diagnosis is the first step towards working with colleagues to find solutions. We are keen to find the best treatment for the NHS over the coming weeks, months and years."
RCP registrar, Dr Andrew Goddard, said: "It is clear to all of us working in the NHS that we are at a point of no return and the NHS in its current form is unsustainable without a significant increase in funding.
"We can't continue to provide ever-more expensive treatments to an ever-increasing group of patients and not expect the system to collapse. As doctors, we see the problems this creates on a daily basis, be it at the front door of the hospital, in A&E or in out-patients."
Dr Johnny Marshall, director of policy at the NHS Confederation, welcomed the report but said care and treatment must be moved closer to people's homes.
Commenting on the report, Dr Mark Porter, chair of council at the British Medical Association, said: "It is clear that the NHS cannot continue to do more with less.
"Patient care is being compromised as hospitals are forced to close their doors due to staffing shortages, waiting times are rising and access to some services is increasingly limited."
Don Redding, director of policy at National Voices - a coalition of health and care charities, added: "We have known for years that rising demand could overwhelm health and care services, and we are now at the crunch point.
"People in different parts of the system are understandably raising the alarm. There is a real need for more money and this autumn is a key time for the new Government to signal that it will respond."
A Department of Health spokesman said: "Staffing is a priority - that's why we have invested in the frontline and there are already more than 24,000 extra clinical staff, including almost 8,800 more doctors and almost 11,300 more nurses, on our wards since May 2010.
"With 50,000 nurses in training we will continue to make sure we have the staff available to give patients high quality care as part of a safer NHS seven days a week."